LibJS: Make it possible to go from a Cell* to its Heap&

This patch makes all HeapBlock allocations aligned to their block size,
enabling us to find the HeapBlock* for a given Cell* by simply masking
bits off of the cell address.

Use this to make a simple Heap& getter for Cell, which lets us avoid
plumbing the Heap& everywhere.

Libraries/LibJS/Cell.cpp

@@ -26,6 +26,7 @@
 
 #include <AK/LogStream.h>
 #include <LibJS/Cell.h>
+#include <LibJS/HeapBlock.h>
 #include <LibJS/Object.h>
 #include <LibJS/PrimitiveString.h>
 #include <LibJS/Value.h>
@@ -38,6 +39,11 @@ void Cell::Visitor::visit(Value value)
         visit(value.as_cell());
 }
 
+Heap& Cell::heap()
+{
+    return HeapBlock::from_cell(this)->heap();
+}
+
 const LogStream& operator<<(const LogStream& stream, const Cell* cell)
 {
     if (!cell)